This invention relates to a process for preparing 1,3-diketone compounds and the products obtained by this process. More particularly the invention relates to the preparation of intermediate compounds in the manufacture of pesticides.
Pesticidal 4-benzoylisoxazoles, particularly 5-cyclopropylisoxazole herbicides and intermediate compounds in their synthesis, are described in the literature, for example in European Patent Publication Nos. 0418175, 0527036, 0560482, 0609798 and 0682659. Various methods for preparing these compounds are known. The present invention seeks to provide an improved method for the preparation of intermediate compounds in their synthesis.
The present invention provides a process for the preparation of a compound of formula (I): 
wherein:
R2 is C1-6 straight- or branched- chain alkylthio, C1-6 alkyl, C1-6 haloalkyl, CH2SR5, C1-6 alkoxy, C1-6 haloalkoxy, or halogen; or a N-linked imidazole, pyrazole, 1,2,3,4-tetrazole, 1,2,3-triazole or 1,2,4-triazole ring which ring systems are optionally substituted by one or more groups selected from halogen. C1-6 alkyl, C1-6 haloalkyl or C1-6 straight- or branched- chain alkylthio;
R3 is C1-6 straight- or branched- chain haloalkyl, C1-6 straight- or branched- chain alkyl, C1-6 alkoxy, C1-6 haloalkoxy, halogen, C1-6 straight- or branched- chain alkylthio or nitro; or a N-linked imidazole, pyrazole, 1,2,3,4-tetrazole, 1,2,3-triazole or 1,2,4-triazole ring which ring systems are optionally substituted by one or more groups selected from halogen, C1-6 alkyl, C1-6 haloalkyl or C1-6 straight- or branched- chain alkylthio.
R4 is hydrogen, C1-6 straight- or branched- chain haloalkyl, C1-6 straight- or branched- chain alkyl, C1-6 alkoxy, C1-6 haloalkoxy, halogen. C1-6 straight- or branched- chain alkylthio-, or a 5 or 6- membered heterocyclic ring (which may be unsaturated or partially saturated) containing 1 to 3 hetero atoms selected from oxygen, nitrogen and sulphur, optionally substituted by halogen. C1-6 haloalkyl, C1-6 alkyl, C1-6 haloalkoxy, C1-6 alkoxy SOnR5, nitro or cyano;
or R3 and R4 together with the carbon atoms to which they are attached, form a 5 to 7 membered saturated or unsaturated heterocyclic ring containing up to three ring heteroatoms selected from nitrogen, oxygen and sulfur, which ring is optionally substituted by one or more groups selected from halogen, nitro, C1-6 straight- or branched- chain alkylthio, C1-4 alkyl, C1-4 alkoxy, C1-4 haloalkyl, C1-4 haloalkoxy, xe2x95x90O and xe2x95x90NOxe2x80x94R5.
R5 represents C1-6 straight- or branched- chain alkyl and
n represents zero, one or two;
which process comprises the reaction of a compound of formula (II): 
wherein R1 is C1-6 straight- or branched- chain alkyl, and R2, R3 and R4 are as hereinbefore defined, with a compound of formula (III): 
in an aprotic solvent in the presence of a base.
When R4 represents a heterocyclic ring, preferred rings include 3-isoxazolyl, 5-isoxazolyl, 2-thiazolyl, 5-oxazolyl, 2-furyl, 3-furyl, 2-thienyl and 3-thienyl.
R1 preferably represents methyl.
R2 preferably represents methylthio or methylthiomethyl.
R3 preferably represents trifluoromethyl or bromo.
R4 preferably represents hydrogen.
In an especially preferred embodiment of the invention R1 represents C1-6 alkyl (methyl is most preferred);
R2 represents methylthiomethyl;
R3 represents bromo; and R4 represents hydrogen.
In a most especially preferred embodiment of the invention R1 represents C1-6 alkyl (methyl is most preferred);
R2 represents C1-6 alkylthio (methylthio is most preferred);
R3 represents C1-6 haloalkyl (trifluoromethyl is most preferred); and R4 represents hydrogen.
Using these reaction conditions it has been found that the reaction gives high yields of the desired final product.
Certain compounds of formula (I) and a number of processes for their preparation have been described in the European Patent Applications cited above. Compounds (II) are known or may be prepared by known methods. Compound (III) and methods for its preparation are known.
The aprotic solvents which may be used may be nonpolar or polar. Nonpolar aprotic solvents which are suitable include aromatic or aliphatic hydrocarbons, particularly toluene and xylenes (toluene is preferred); aromatic halogenated hydrocarbons (chlorobenzene is preferred) or ethers such as tert-butyl methyl ether, dioxan or tetrahydrofuran. Examples of suitable polar aprotic solvents include dimethylsulfoxide, N-methylpyrrolidinone, N,N-dimethylformamide or N,N-dimethylacetamide (dimethylsulfoxide and N-methylpyrrolidinone are preferred). Chlorobenzene and toluene are especially preferred because of the good results obtained, and cost and suitability of such solvents for large scale preparations.
Generally the reaction temperature used in the above process is from about 0xc2x0 C. to the boiling point of the solvent, preferably from about 0xc2x0 C. to about 100xc2x0 C.; for non-polar solvents such as toluene or chlorobenzene a temperature of from about 40xc2x0 C. to about 90xc2x0 C. is especially preferred; whilst for polar aprotic solvents such as dimethylsulfoxide and N-methylpyrrolidinone a temperature of from about 20xc2x0 C. to about 40xc2x0 C. is especially preferred.
Generally the reaction takes place in the presence of a strong base which is most preferably selected from an alkoxide of an alkali or alkaline earth metal, notably sodium ethoxide, sodium methoxide, sodium or potassium t-butoxide; and a metal hydride (notably sodium hydride). Sodium methoxide is an especially preferred base for reasons of effectiveness in the reaction.
According to a preferred variant of the process of the present invention the reaction is performed with continuous distillation of the alcohol R1xe2x80x94OH formed in the course of the reaction, at atmospheric pressure or under reduced pressure (preferably from 150 to 250 mbars). Alternatively the alcohol R1xe2x80x94OH formed may be removed by the use of a suitable molecular sieve, for example a 4 Angstrom molecular sieve.
Compounds of formula (II) wherein R2 represents C1-6 alkylthio; R3 represents C1-6 haloalkyl and R4 represents hydrogen may be prepared by the reaction of a compound of formula (IV): 
wherein R1 and R3 are as defined above and R6 represents a halogen atom selected from fluorine chlorine and bromine (preferably fluorine or chlorine), with an alkylthiol (or metal salt thereof) of formula R2xe2x80x94X wherein R2 is as defined above (preferably methylthio) and X represents hydrogen or an alkali metal (preferably sodium, potassium or lithium).
Preferably the above reaction to prepare intermediates of formula (II) is performed under substantially anhydrous conditions using a polar aprotic solvent preferably N-methylpyrrolidinone, acetonitrile or dimethylsulphoxide, at a temperature of from about xe2x88x9220xc2x0 to about 150xc2x0 C., preferably 0xc2x0 to about 50xc2x0 C.
The following non-limiting examples illustrate the invention.